Ukraine's interim president Olexander Turchynov ordered the military pullout on Monday morning because of "Russian threats to the lives of military staff and their families".

His announcement came shortly after Russian forces seized a naval base at Feodosia - the third such takeover in 48 hours.

The BBC's Ian Pannell, in the Crimean capital of Simferopol, said he saw Ukrainian marines heading to the border from the naval base.

Ukrainian Defence Ministry spokesman Vladislav Seleznyov said Russians had attacked the base from two directions using armoured personnel carriers and stun grenades. Two Ukrainian navy minesweepers were also stormed earlier on Monday by Russian forces, reports said.

Russian defence officials earlier said that the tricolour of Russia had been hoisted at 189 Ukrainian military units and facilities in Crimea.

Widespread defections

Meanwhile Russian Defence Minister Sergei Shoigu has met troops in Crimea and inspected military bases. He is the most senior Russian official to visit the region since it was annexed by Moscow.

Mr Shoigu also rewarded the former head of Ukraine's navy, Rear Admiral Denys Berezovsky - one of few Ukrainian officers to switch allegiances before the annexation - with the deputy command of Russia's Black Sea Fleet.

Since annexation, many more troops are believed to have defected.

In a press conference, Oleksandr Rozmaznin, deputy chief of the Ukrainian armed forces' general staff, said: "We know the names of almost all the commanders who have signed a contract with the Russian Federation.

"I won't tell you their names as they don't deserve being mentioned, but they represent around 50 percent."

Despite the news that Ukrainian troops were already on the move, he also insisted Ukraine needed guarantees in order to pull its troops out.

"No military unit will leave without a corridor because if there are no agreements there will be fighting, and we are not considering to start a fight or use weapons," Mr Rozmaznin said.

"Therefore we are now in the process of negotiations."

"Imposing a cost on Russia"

The G7 group of industrialised countries, meeting in The Hague in the Netherlands, is considering a collective response to the crisis.

G7 leaders are meeting on the sidelines of a long-planned summit on global threats to nuclear security.

Russian Foreign Minister Sergei Lavrov is expected to have talks with his Ukrainian counterpart, Andriy Deshchytsya. He will also meet US Secretary of State John Kerry.

Speaking ahead of the talks, US President Barack Obama said Europe and America were united in their support of the Ukrainian government and its people.

Alongside Dutch Prime Minister Mark Rutte, Mr Obama said the US and Europe were also "united in imposing a cost on Russia for its actions (in Ukraine) so far".

Russian soldiers assumed an increasingly high profile in Feodosia in recent days

Barack Obama and other G7 leaders are discussing the Ukraine crisis on the sidelines of a Dutch summit

Pro-Russian protesters continue to rally in the eastern city of Donetsk

Nato's military commander in Europe warned on Sunday that Russian forces on Ukraine's eastern borders were capable of mounting an operation all the way to Moldova.

In other developments:

Several parts of Crimea were hit by power cuts on Sunday evening, which officials blamed on technical problems

There is still no confirmation of the whereabouts of Col Yuli Mamchur, the commander of Belbek base, which fell on Saturday. He was taken by Russian forces reportedly to a military prison. The interim Ukrainian president has demanded his immediate release

The rouble entered force as Crimea's official currency on Monday but the Ukrainian hryvnia remains in use until 1 January 2016, the pro-Russian Crimean prime minister says

Moscow's annexation of Crimea on 16 March came after protesters overthrew pro-Kremlin President Viktor Yanukovych in February.

Russia said it had acted to protect its "compatriots" in Crimea from "fascists" moving in from mainland Ukraine.

The US and EU have responded with a series of sanctions targeting those individuals including senior officials whom they accuse of involvement in Crimea's annexation.